I am not not
absolutely sure of anything. I am
relatively confident in that statement, because it seems to be the consensus on the wikis and other sources I've read, and it makes sense wrt how -res mods work in general. I adduce a few refs down below.
You get an infinity for the portable, powerful Conviction Aura, that benefits you greatly, for all elements, but tends to benefit lightning more, because L-immunes statistically have lower resistances than Fire and Cold immunes. They are "less strongly immune", if you will.
Paladins with their own Conviction aura observe the same.
-res on items, like -res in Cold Mastery, benefits the person with the item / the skill, and only they. The "X immune" mention does not disappear with them, even if the monster can be damaged, because the monster is still immune
for other people.
You get a griffon's eye
for your sorc because it increases
her lightning damage (assuming not-immune/broken monster, and not already at -100% cap).
On you merc, tough? It won't do anything for
you.
A few refs:
Infinity is the rune word 'BerMalBerIst' for polearms in Diablo II: Lord of Destruction. The Conviction aura provided by Infinity maximizes the effects of the high-level Chain Lightning that the weapon produces. For nearly every character that deals elemental damage, this is the best mercenary...
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I played solo with occasional TCP/IP with my mother for a loooong time (on and off). I think I started using item editors with patch 1.10 or whatever it was that made Hell 10 times more difficult, with immunes everywhere. Not good for sorc especially.
I posted a charm earlier on this thread that boosts Magic Find for solo play. You may enjoy it as a more "legit" way of finding (slightly) better items in solo play, as opposed to editing them in or buying them (the latter of which I've never done). It's equivalent to a free MF-build.